This invention relates to anionic polymerization using protected functional initiators.
Anionic polymerization using an alkali metal initiator is well known. Generally, an alkyl lithium such as secondary butyl lithium is used. It is sometimes desirable to incorporate functional groups at the ends of polymer chains, particularly elastomeric polymer chains. Polymers formed from monolithium initiators can be terminated with ethylene oxide to give an --OLi structure at one end which can be utilized for incorporating a functional group, but this leaves the other end without a functional group. The incorporation of functional groups can be done with dilithium initiators simply by adding a terminating agent such as ethylene oxide which gives terminal lithium-oxygen structures at each end of the polymer. However, such polymers tend to form gels.
Recently, a technique has been developed to provide functional groups at each end, utilizing what is called a protected functional initiator (PFI). At the end of the polymerization the protective group is removed (deprotection) to give the desired terminal functional group. While the protecting group can be removed to give the functionality, it is somewhat difficult to practice and is costly. Deprotection of polymers of this type required contacting with a molar excess (5.times. stoichiometry) of a strong organic acid, such as methanesulfonic acid, and a compatibilizing cosolvent such as isopropanol (about 20% wt). This mixture is then stirred at elevated temperatures (about 50.degree. C.) until the polymer is deprotected (several hours depending on the specific initiator that is used). When the polymer has been deprotected, it is then necessary to neutralize the acidic hydrolysis catalyst, wash out the spent acid salt, and distill out the compatibilizing cosolvent. These additional steps add time and cost to the process.
At the conclusion of any anionic polymerization using an alkali metal catalyst, the metal remains in the reaction medium. Also many end uses require hydrogenation of the polymer to remove nonaromatic unsaturation which results in the presence of metallic hydrogenation catalyst residues, all of which must be removed before the polymer can be utilized.
Filtration has long been known as a method of removing insoluble metal contaminants. However, filtration of viscous polymer solutions is generally slow and expensive. It is well known to extract metal with aqueous solutions of acids or chelates. This approach is frequently unsatisfactory because of emulsion formation, requirements of extended settling times, or the saturation of the organic phase with water. Also polymers, particularly elastomers, have limits on how harshly they can be treated without decomposing.